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class="post-meta-date"><i class="far fa-calendar-alt fa-fw post-meta-icon"></i><span class="post-meta-label">发表于</span><time class="post-meta-date-created" datetime="2020-10-15T03:22:57.000Z" title="发表于 2020-10-15 11:22:57">2020-10-15</time><span class="post-meta-separator">|</span><i class="fas fa-history fa-fw post-meta-icon"></i><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2020-10-15T07:04:05.884Z" title="更新于 2020-10-15 15:04:05">2020-10-15</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0/">学习笔记</a></span></div><div class="meta-secondline"> <span class="post-meta-separator">|</span><span class="post-meta-wordcount"><i class="far fa-file-word fa-fw post-meta-icon"></i><span class="post-meta-label">字数总计:</span><span class="word-count">2.4k</span><span class="post-meta-separator">|</span><i class="far fa-clock fa-fw post-meta-icon"></i><span class="post-meta-label">阅读时长:</span><span>9分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"></span></span></div></div></div></header><main class="layout" id="content-inner"><div id="post"><article class="post-content" id="article-container"><h1 id="ES6关于Promise的用法"><a href="#ES6关于Promise的用法" class="headerlink" title="ES6关于Promise的用法"></a>ES6关于Promise的用法</h1><blockquote>
<p>转自：<a target="_blank" rel="noopener" href="https://segmentfault.com/a/1190000011652907">https://segmentfault.com/a/1190000011652907</a></p>
</blockquote>
<ul>
<li>Node的产生，大大推动了<code>Javascript</code>这门语言在服务端的发展，使得前端人员可以以很低的门槛转向后端开发。</li>
<li>当然，这并不代表迸发成了<em>全栈</em>。全栈的技能很集中，绝不仅仅是前端会写一些<code>HTML</code>和一些<em>交互</em>，后台熟悉<em>数据库的增删查改</em>。</li>
<li>想必接触过Node的人都知道，Node是以<em>异步<code>(Async)</code>回调</em>著称的，其异步性提高了程序的执行效率，但同时也减少了程序的可读性。如果我们有几个异步操作，并且后一个操作需要前一个操作返回的数据才能执行，这样按照Node的一般执行规律，要实现有序的异步操作，通常是一层加一层嵌套下去。</li>
<li>为了解决这个问题，ES6提出了<code>Promise</code>的实现。</li>
</ul>
<hr>
<ul>
<li><h2 id="定义"><a href="#定义" class="headerlink" title="定义:"></a>定义:</h2><blockquote>
<p><em>Promise 对象用于一个异步操作的最终完成（或失败）及其结果值的表示</em>。简单点说，它就是用于<strong>处理异步操作</strong>的，异步处理成功了就执行成功的操作，异步处理失败了就捕获错误或者停止后续操作。</p>
</blockquote>
<ul>
<li><h4 id="它的一般表示形式为："><a href="#它的一般表示形式为：" class="headerlink" title="它的一般表示形式为："></a>它的一般表示形式为：</h4><figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="keyword">new</span> <span class="built_in">Promise</span>(</span><br><span class="line">    <span class="comment">/* executor */</span></span><br><span class="line">    <span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">        <span class="keyword">if</span> (<span class="comment">/* success */</span>) &#123;</span><br><span class="line">            <span class="comment">// ...执行代码</span></span><br><span class="line">            resolve();</span><br><span class="line">        &#125; <span class="keyword">else</span> &#123; <span class="comment">/* fail */</span></span><br><span class="line">            <span class="comment">// ...执行代码</span></span><br><span class="line">            reject();</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">);</span><br></pre></td></tr></table></figure>

<ul>
<li>其中，<code>Promise</code>中的参数<code>executor</code>是一个<strong>执行器函数</strong>，它有两个参数<code>resolve</code>和<code>reject</code>。它内部通常有一些异步操作，如果异步操作成功，则可以调用resolve()来将该实例的状态置为<code>fulfilled</code>，即已完成的，如果一旦失败，可以调用<code>reject()</code>来将该实例的状态置为<code>rejected</code>，即失败的。</li>
<li>我们<strong>可以把Promise对象看成是一条工厂的流水线</strong>，对于流水线来说，从它的工作职能上看，它只有三种状态，一个是初始状态（刚开机的时候），一个是加工产品成功，一个是加工产品失败（出现了某些故障）。同样对于Promise对象来说，它也有<strong>三种状态</strong>：<ol>
<li><code>pending</code><br><strong>初始状态,也称为未定状态</strong>，就是初始化Promise时，调用executor执行器函数后的状态。</li>
<li><code>fulfilled</code><br><strong>完成状态</strong>，意味着异步操作成功。</li>
<li><code>rejected</code><br><strong>失败状态</strong>，意味着异步操作失败。</li>
</ol>
</li>
<li>并且这个状态转化是<strong>单向的，不可逆转</strong>，已经确定的状态（fulfilled/rejected）无法转回初始状态（pending）。</li>
</ul>
</li>
</ul>
<hr>
</li>
<li><h2 id="方法："><a href="#方法：" class="headerlink" title="方法："></a>方法：</h2><ul>
<li><h4 id="Promise-prototype-then"><a href="#Promise-prototype-then" class="headerlink" title="Promise.prototype.then()"></a><code>Promise.prototype.then()</code></h4><ul>
<li><p><code>Promise</code>对象含有<code>then</code>方法，then()调用后返回一个Promise对象，意味着实例化后的Promise对象可以进行<strong>链式调用</strong>，而且这个then()方法可以接收两个函数，一个是处理成功后的函数，一个是处理错误结果的函数。</p>
</li>
<li><h4 id="如下："><a href="#如下：" class="headerlink" title="如下："></a>如下：</h4><figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="keyword">var</span> promise1 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 2秒后置为接收状态</span></span><br><span class="line">  <span class="built_in">setTimeout</span>(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    resolve(<span class="string">&#x27;success&#x27;</span>);</span><br><span class="line">  &#125;, <span class="number">2000</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line">promise1.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// success</span></span><br><span class="line">&#125;, <span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(err); <span class="comment">// 不执行</span></span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 上一步的then()方法没有返回值</span></span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;链式调用：&#x27;</span> + data); <span class="comment">// 链式调用：undefined </span></span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// ....</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>
</li>
<li><p>在这里我们主要关注<code>promise1.then()</code>方法调用后返回的Promise对象的状态，是<code>pending</code>还是<code>fulfilled</code>，或者是<code>rejected</code>?</p>
</li>
<li><p><strong>返回的这个<code>Promise</code>对象的状态主要是根据promise1.then()方法返回的值</strong>，大致分为以下几种情况：</p>
<ol>
<li>如果then()方法中返回了一个参数值，那么返回的Promise将会变成接收状态。</li>
<li>如果then()方法中抛出了一个异常，那么返回的Promise将会变成拒绝状态。</li>
<li>如果then()方法调用resolve()方法，那么返回的Promise将会变成接收状态。</li>
<li>如果then()方法调用reject()方法，那么返回的Promise将会变成拒绝状态。</li>
<li>如果then()方法返回了一个未知状态(pending)的Promise新实例，那么返回的新Promise就是未知状态。</li>
<li>如果then()方法没有明确指定的<code>resolve(data)/reject(data)/return data</code>时，那么返回的新Promise就是接收状态，可以一层一层地往下传递。</li>
</ol>
</li>
<li><h4 id="转换实例如下："><a href="#转换实例如下：" class="headerlink" title="转换实例如下："></a>转换实例如下：</h4><figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="keyword">var</span> promise2 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 2秒后置为接收状态</span></span><br><span class="line">  <span class="built_in">setTimeout</span>(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    resolve(<span class="string">&#x27;success&#x27;</span>);</span><br><span class="line">  &#125;, <span class="number">2000</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line">promise2</span><br><span class="line">  .then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">    <span class="comment">// 上一个then()调用了resolve，置为fulfilled态</span></span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;第一个then&#x27;</span>);</span><br><span class="line">    <span class="built_in">console</span>.log(data);</span><br><span class="line">    <span class="keyword">return</span> <span class="string">&#x27;2&#x27;</span>;</span><br><span class="line">  &#125;)</span><br><span class="line">  .then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">    <span class="comment">// 此时这里的状态也是fulfilled, 因为上一步返回了2</span></span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;第二个then&#x27;</span>);</span><br><span class="line">    <span class="built_in">console</span>.log(data);  <span class="comment">// 2</span></span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">      reject(<span class="string">&#x27;把状态置为rejected error&#x27;</span>); <span class="comment">// 返回一个rejected的Promise实例</span></span><br><span class="line">    &#125;);</span><br><span class="line">  &#125;, <span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">    <span class="comment">// error</span></span><br><span class="line">  &#125;)</span><br><span class="line">  .then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">    <span class="comment">/* 这里不运行 */</span></span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;第三个then&#x27;</span>);</span><br><span class="line">    <span class="built_in">console</span>.log(data);</span><br><span class="line">    <span class="comment">// ....</span></span><br><span class="line">  &#125;, <span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">    <span class="comment">// error回调</span></span><br><span class="line">    <span class="comment">// 此时这里的状态也是fulfilled, 因为上一步使用了reject()来返回值</span></span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;出错：&#x27;</span> + err); <span class="comment">// 出错：把状态置为rejected error</span></span><br><span class="line">  &#125;)</span><br><span class="line">  .then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">    <span class="comment">// 没有明确指定返回值，默认返回fulfilled</span></span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;这里是fulfilled态&#x27;</span>);</span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<hr>
</li>
<li><h4 id="Promise-prototype-catch"><a href="#Promise-prototype-catch" class="headerlink" title="Promise.prototype.catch():"></a><code>Promise.prototype.catch()</code>:</h4><ul>
<li><p><code>catch()</code>方法和<code>then()</code>方法一样，都会返回一个新的Promise对象，它主要<strong>用于捕获异步操作时出现的异常</strong>。因此，我们通常省略then()方法的第二个参数，把错误处理控制权转交给其后面的catch()函数，如下：</p>
<figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="keyword">var</span> promise3 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">setTimeout</span>(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    reject(<span class="string">&#x27;reject&#x27;</span>);</span><br><span class="line">  &#125;, <span class="number">2000</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line">promise3.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;这里是fulfilled状态&#x27;</span>); <span class="comment">// 这里不会触发</span></span><br><span class="line">  <span class="comment">// ...</span></span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 最后的catch()方法可以捕获在这一条Promise链上的异常</span></span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;出错：&#x27;</span> + err); <span class="comment">// 出错：reject</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<hr>
</li>
<li><h4 id="Promise-all"><a href="#Promise-all" class="headerlink" title="Promise.all():"></a><code>Promise.all()</code>:</h4><ul>
<li><p><code>Promise.all()</code>接收一个参数，它必须是<strong>可以迭代的，比如数组</strong>。</p>
</li>
<li><p>它通常用来处理一些<strong>并发的异步操作</strong>，即它们的结果互不干扰，但是又需要异步执行。它最终只有两种状态：<strong>成功或者失败</strong>。</p>
</li>
<li><p>它的状态受参数内各个值的状态影响，即里面状态全部为<code>fulfilled</code>时，它才会变成<code>fulfilled</code>，否则变成<code>rejected</code>。</p>
</li>
<li><p>成功调用后<strong>返回一个数组</strong>，数组的值是<strong>有序</strong>的，即按照传入参数的数组的值操作后返回的结果。如下：</p>
<figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="comment">// 置为fulfilled状态的情况</span></span><br><span class="line"><span class="keyword">var</span> arr = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];</span><br><span class="line"><span class="keyword">var</span> promises = arr.map(<span class="function"><span class="keyword">function</span>(<span class="params">e</span>) </span>&#123;</span><br><span class="line">  <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">    resolve(e * <span class="number">5</span>);</span><br><span class="line">  &#125;);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.all(promises).then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">    <span class="comment">// 有序输出</span></span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// [5, 10, 15]</span></span><br><span class="line">  <span class="built_in">console</span>.log(arr); <span class="comment">// [1, 2, 3]</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>

<figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="comment">// 置为rejected状态的情况</span></span><br><span class="line"><span class="keyword">var</span> arr = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];</span><br><span class="line"><span class="keyword">var</span> promises2 = arr.map(<span class="function"><span class="keyword">function</span>(<span class="params">e</span>) </span>&#123;</span><br><span class="line">  <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (e === <span class="number">3</span>) &#123;</span><br><span class="line">      reject(<span class="string">&#x27;rejected&#x27;</span>);</span><br><span class="line">    &#125;</span><br><span class="line">    resolve(e * <span class="number">5</span>);</span><br><span class="line">  &#125;);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.all(promises2).then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 这里不会执行</span></span><br><span class="line">  <span class="built_in">console</span>.log(data);</span><br><span class="line">  <span class="built_in">console</span>.log(arr);</span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(err); <span class="comment">// rejected</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<hr>
</li>
<li><h4 id="Promise-race"><a href="#Promise-race" class="headerlink" title="Promise.race():"></a><code>Promise.race()</code>:</h4><ul>
<li><p><code>Promise.race()</code>和<code>Promise.all()</code>类似，都接收一个<strong>可以迭代的参数</strong>，但是不同之处是<code>Promise.race()</code>的状态变化不是全部受参数内的状态影响，一旦参数内有一个值的状态发生的改变，那么该Promise的状态就是改变的状态。就跟<code>race</code>单词的字面意思一样，谁跑的快谁赢。如下：</p>
<figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="keyword">var</span> p1 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">setTimeout</span>(resolve, <span class="number">300</span>, <span class="string">&#x27;p1 doned&#x27;</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> p2 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">setTimeout</span>(resolve, <span class="number">50</span>, <span class="string">&#x27;p2 doned&#x27;</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> p3 = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">setTimeout</span>(reject, <span class="number">100</span>, <span class="string">&#x27;p3 rejected&#x27;</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="built_in">Promise</span>.race([p1, p2, p3]).then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 显然p2更快，所以状态变成了fulfilled</span></span><br><span class="line">  <span class="comment">// 如果p3更快，那么状态就会变成rejected</span></span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// p2 doned</span></span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(err); <span class="comment">// 不执行</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<hr>
</li>
<li><h4 id="Promise-resolve"><a href="#Promise-resolve" class="headerlink" title="Promise.resolve():"></a><code>Promise.resolve()</code>:</h4><ul>
<li><p><code>Promise.resolve()</code>接受一个参数值，可以是<code>普通的值</code>，<code>具有then()方法的对象</code>和<code>Promise实例</code>。正常情况下，它返回一个Promise对象，状态为<code>fulfilled</code>。但是，当解析时发生错误时，返回的Promise对象将会置为<code>rejected</code>态。如下：</p>
<figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="comment">// 参数为普通值</span></span><br><span class="line"><span class="keyword">var</span> p4 = <span class="built_in">Promise</span>.resolve(<span class="number">5</span>);</span><br><span class="line">p4.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// 5</span></span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">// 参数为含有then()方法的对象</span></span><br><span class="line"><span class="keyword">var</span> obj = &#123;</span><br><span class="line">  then: <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">&#x27;obj 里面的then()方法&#x27;</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> p5 = <span class="built_in">Promise</span>.resolve(obj);</span><br><span class="line">p5.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 这里的值时obj方法里面返回的值</span></span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// obj 里面的then()方法</span></span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">// 参数为Promise实例</span></span><br><span class="line"><span class="keyword">var</span> p6 = <span class="built_in">Promise</span>.resolve(<span class="number">7</span>);</span><br><span class="line"><span class="keyword">var</span> p7 = <span class="built_in">Promise</span>.resolve(p6);</span><br><span class="line"></span><br><span class="line">p7.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 这里的值时Promise实例返回的值</span></span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// 7</span></span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 参数为Promise实例,但参数是rejected态</span></span><br><span class="line"><span class="keyword">var</span> p8 = <span class="built_in">Promise</span>.reject(<span class="number">8</span>);</span><br><span class="line"><span class="keyword">var</span> p9 = <span class="built_in">Promise</span>.resolve(p8);</span><br><span class="line"></span><br><span class="line">p9.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 这里的值是Promise实例返回的值</span></span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;fulfilled:&#x27;</span>+ data); <span class="comment">// 不执行</span></span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;rejected:&#x27;</span> + err); <span class="comment">// rejected: 8</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<hr>
</li>
<li><h4 id="Promise-reject"><a href="#Promise-reject" class="headerlink" title="Promise.reject():"></a><code>Promise.reject()</code>:</h4><ul>
<li><p><code>Promise.reject()</code>和<code>Promise.resolve()</code>正好相反，它接收一个参数值<code>reason</code>，即<strong>发生异常的原因</strong>。此时返回的Promise对象将会置为<code>rejected</code>态。如下：</p>
<figure class="highlight javascript"><table><tr><td class="code"><pre><span class="line"><span class="keyword">var</span> p10 = <span class="built_in">Promise</span>.reject(<span class="string">&#x27;手动拒绝&#x27;</span>);</span><br><span class="line">p10.then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(data); <span class="comment">// 这里不会执行，因为是rejected态</span></span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span>(<span class="params">err</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(err); <span class="comment">// 手动拒绝</span></span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span>(<span class="params">data</span>) </span>&#123;</span><br><span class="line"> <span class="comment">// 不受上一级影响</span></span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;状态：fulfilled&#x27;</span>); <span class="comment">// 状态：fulfilled</span></span><br><span class="line">&#125;);</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>总之，除非<code>Promise.then()</code>方法内部抛出异常或者是明确置为<code>rejected</code>态，否则它返回的Promise的状态都是fulfilled态，即完成态，并且它的状态不受它的上一级的状态的影响。</strong></p>
</li>
</ul>
</li>
</ul>
<hr>
</li>
<li><h2 id="总结："><a href="#总结：" class="headerlink" title="总结："></a>总结：</h2><ul>
<li>大概常用的方法就写那么多，剩下的看自己实际需要再去了解。</li>
<li>解决Node回调地狱的不止有<code>Promise</code>，还有<code>Generator</code>和ES7提出的<code>Async</code>实现。</li>
<li><strong>方法不在多，而在于精</strong>。</li>
</ul>
</li>
</ul>
<hr>
</article><div class="post-copyright"><div class="post-copyright__author"><span class="post-copyright-meta">文章作者: </span><span class="post-copyright-info"><a href="mailto:undefined">六根清静</a></span></div><div class="post-copyright__type"><span class="post-copyright-meta">文章链接: </span><span class="post-copyright-info"><a href="https://liugenqingjing2.coding.me/2020/10/15/%E8%A7%A3%E8%AF%BBPromise-%E6%90%AC%E8%BF%90%E5%A4%A7%E4%BD%AC%E7%9A%84/">https://liugenqingjing2.coding.me/2020/10/15/解读Promise-搬运大佬的/</a></span></div><div class="post-copyright__notice"><span class="post-copyright-meta">版权声明: </span><span class="post-copyright-info">本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/4.0/" target="_blank">CC BY-NC-SA 4.0</a> 许可协议。转载请注明来自 <a href="https://liugenqingjing2.coding.me" target="_blank">小何blog (记录美好)</a>！</span></div></div><div class="tag_share"><div class="post-meta__tag-list"><a class="post-meta__tags" href="/tags/JavaScript/">JavaScript</a><a 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href="#%E5%AE%83%E7%9A%84%E4%B8%80%E8%88%AC%E8%A1%A8%E7%A4%BA%E5%BD%A2%E5%BC%8F%E4%B8%BA%EF%BC%9A"><span class="toc-number">1.1.0.1.</span> <span class="toc-text">它的一般表示形式为：</span></a></li></ol></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%96%B9%E6%B3%95%EF%BC%9A"><span class="toc-number">1.2.</span> <span class="toc-text">方法：</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#Promise-prototype-then"><span class="toc-number">1.2.0.1.</span> <span class="toc-text">Promise.prototype.then()</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E5%A6%82%E4%B8%8B%EF%BC%9A"><span class="toc-number">1.2.0.2.</span> <span class="toc-text">如下：</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E8%BD%AC%E6%8D%A2%E5%AE%9E%E4%BE%8B%E5%A6%82%E4%B8%8B%EF%BC%9A"><span class="toc-number">1.2.0.3.</span> <span class="toc-text">转换实例如下：</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Promise-prototype-catch"><span class="toc-number">1.2.0.4.</span> <span class="toc-text">Promise.prototype.catch():</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Promise-all"><span class="toc-number">1.2.0.5.</span> <span class="toc-text">Promise.all():</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Promise-race"><span class="toc-number">1.2.0.6.</span> <span class="toc-text">Promise.race():</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Promise-resolve"><span class="toc-number">1.2.0.7.</span> <span class="toc-text">Promise.resolve():</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Promise-reject"><span class="toc-number">1.2.0.8.</span> <span class="toc-text">Promise.reject():</span></a></li></ol></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%80%BB%E7%BB%93%EF%BC%9A"><span class="toc-number">1.3.</span> <span class="toc-text">总结：</span></a></li></ol></li></ol></div></div></div><div class="card-widget card-recent-post"><div class="card-content"><div class="item-headline"><i class="fas fa-history"></i><span>最新文章</span></div><div class="aside-list"><div class="aside-list-item"><a class="thumbnail" href="/2021/03/21/Node-js%E5%AD%A6%E4%B9%A0-%E4%BA%8C%E5%8D%81%E4%B8%89-%E8%B7%A8%E5%9F%9F%E4%BB%8B%E7%BB%8D%E5%92%8CJSONP%E4%BB%8B%E7%BB%8D/" title="Node.js学习(二十三.跨域介绍和JSONP介绍)"><img data-lazy-src="https://gitee.com/he_chaoming/blog_tuchuang/raw/master/20210321/qWPEashJOuNK.jpg" onerror="this.onerror=null;this.src='/img/404.jpg'" alt="Node.js学习(二十三.跨域介绍和JSONP介绍)"/></a><div class="content"><a class="title" href="/2021/03/21/Node-js%E5%AD%A6%E4%B9%A0-%E4%BA%8C%E5%8D%81%E4%B8%89-%E8%B7%A8%E5%9F%9F%E4%BB%8B%E7%BB%8D%E5%92%8CJSONP%E4%BB%8B%E7%BB%8D/" title="Node.js学习(二十三.跨域介绍和JSONP介绍)">Node.js学习(二十三.跨域介绍和JSONP介绍)</a><time datetime="2021-03-21T13:40:58.000Z" title="发表于 2021-03-21 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